Travelling wave device



Dec. 18, 1962 c, DENCH 3,069,587

TRAVELLING WAVE DEVICE Filed Sept. 24, 1953 /2 F INPUT 3 Sheets-Sheet 2 'f RF ourpur FIG. 6

me. 7 N M g w y H lNvENTOl? EDWARD C DENCH TORNEY Dec. 18, 1962 E. c. DENCH TRAVELLING WAVE DEVICE 3 Sheets-Sheet 3 Filed Sept. 24, 1955 INVENTOR EDWARD C. DENCH BY (22...

AT /2 EV nits This invention involves a means for introducing attenuation in electronic tubes of the traveling wave type.

In electron discharge devices of the traveling wave type, it is often necessary to insert attenuation in some part of the tube, usually in the anode delay line. In the case of traveling wave amplifiers, such attenuation is inserted between the input and output coupling means and serves to prevent oscillations resulting from reflections of energy from the load at the output end of the tube back to the input end in the case of an imperfect impedance match at the output. In the traveling wave oscillator, the aforesaid attenuation is inserted at the end of the tube remote from the output end in order to prevent frequency discontinuities in the oscillator during tuning.

Previously the attenuation has been provided by a thin coating of lossy material, such as graphite mixed with a suitable binder and applied to the electrical field propagating structure of the tube, either by dipping or spraying. Another possibility is the application of a coating of iron by electroplating techniques. The amount of attenuation achieved by coating the periodic structure of the tube has been found inadequate unless the length of the coated portion f the tube structure, and therefore the total length of the tube, is made unduly great. For reasons of economy, space saving and efficiency of operation, the length of the region of attenuation within the tube should be a minimum.

Although certain materials recently have been developed which are somewhat more lossy than graphite, these alloys are not practical for use in traveling wave tubes since the materials are difficult to apply uniformly and the adhesion of the material to the periodic delay structure is poor.

Pursuant to this invention, at least one of the dimensions of the periodic anode delay structure of the traveling wave tube is progressively altered over the region in which attenuation is to be introduced. In this way, the attenuation is progressively increased in the direction of travel of the backward wave, to be defined subsequently, in the case of the amplifier and in the direction of travel of the forward wave, also defined below, in the case of the oscillator. By progressively altering the aforesaid dimensions, a point along the attenuation region will always occur for some frequency in the pass band where cutofi is approached and the attenuation approaches infinity.

By means of this invention, greatly improved attenuation for a unit length may be achieved; furthermore, the amount of attenuation is more readily reproducible in applicants technique than with the presently available spraying or plating techniques.

The operation of devices of the traveling wave type is based upon the interaction between an electromagnetic wave and an electron beam, both of which are traveling along the discharge device at substantially the same speed. When the electrons remain in step with the electromagnetic wave, a progressive density modulation of the beam is obtained such that energy is transferred from the electron beam to the electromagnetic field. In order to achieve effective energy transfer from the electron space charge field to the electromagnetic field, it is necessary to retard the electromagnetic wave propagating along the electron discharge device. This may be accomplished by means of a periodically loaded delay line, more simply referred to as a periodic structure,

3,069,587 Patented Dec. 18, 1962 ice over which the electromagnetic wave is allowed to propagate. A periodic structure may be defined as a transmission line consisting of successive identical sections which are similarly arranged but whose electrical properties are not uniform throughout.

A periodic structure of which the .interdigital line, strapped vane network or loaded wave guide are examples, has the properties of a band pass filter network. The electromagnetic field associated with such periodic structures does not vary sinusoidally with distance but has a periodic distribution and this field can be analyzed as a series of sinusoidal field components. The field along a periodic structure thus appears as though it were a superposition of traveling waves or space harmonics having a phase velocity V given by where A is the pitch or length of one network section, is the phase shift per section and n is the number of the space harmonic.

In filter circuits ,of this type there is an infinite number of space harmonics some of which have positive phase velocities and some negative. When V has a positive value, that is, when 0 yb 2n3 the phase velocity is in the same direction as the energy or group velocity and the corresponding waves will be referred to as forward waves. When V is negative, that is, when n 0, the phase velocity is in a direction opposite to the energy velocity and the corresponding waves are referred to as backward waves. When the electron beam velocity equals the phase velocity of some forward Wave, energy carried by the line increases in the direction of motion of the electron and the device functions as an amplifier. Whenan electron velocity is synchronized with that of a backward wave, however, the energy of which propagates in a direction opposite to that of the beam, the device serves as an oscillator.

In one type of traveling wave electron discharge device, the electron beams move only in an electric field associated with the periodic delay line. In another type of tube, the beam also moves through a magnetic field arranged transversely to said electric field. Either type may be used as an amplifier or as an oscillator, depending upon the space harmonic with which the electron beam is synchronized.

The invention is applicable to any of the types of traveling wave tubes described above.

The foregoing and other features of the invention will become more apparent from the detailed description of certain specific embodiments which follows. The description refers to the accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view of a first embodiment of this invention having an interdigital anode structure and adapted for use as a traveling wave oscillator;

FIG. 2 is a fragmentary sectional View of the interdigital anode structure taken along line 22 of FIG. 1;

FIG. 3 is a sectional view of a portion of the interdigital anode structure taken along line 33 of FIG. 1;

FIG. 4 is an enlarged view of a portion of an interdigital anode structure of FIG. 1;

FIG. 5 is an enlarged view of a modification of the interdigital anode structure shown in FIG. 4;

FIG. 6 is a longitudinal cross-sectional view of a second embodiment of this invention adapted for use as a traveling wave amplifier;

FIG. 7 is a fragmentary enlarged view of a modification of the tapered portion of the anode structure of FIG. 6;

FIG. 8 is a transverse cross-sectional view of a third ano es? embodiment of this invention having a loaded wave guide anode structure;

FIG. 9 is a fragmentary longitudinal view of a portion of the wave guide of FIG. 8;

FIG. 10 is an isometric view of the loaded wave guide periodic anode structure used in FIG. 8;

FIGS. 11 and 12 are curves illustrating certain operating characteristics of the various embodiments of this in vention;

FIG. 13 is a fragmentary view of a portion of a fourth embodiment of this invention;

FIG. 14 is a cross-sectional View of a fifth embodiment of this invention in which the periodic structure is arcuate rather than linear; and

FIG. 15 is a detailed view of a portion of the device shown in FIG. 14.

Since the phase of velocity of the forward and backward waves is an important factor in determining the interaction between an electron beam and the electromagnetic field of a periodic structure, it is convenient to represent graphically the properties of a periodic structure by the variation of c/V the ratio of the velocity of light to the phase velocity V of the space harmonies, against wave length A.

Typical curves are shown in FIGS. 11 and 12 of the drawings. These curves are symmetrical about a straight line D of constant 1/ passing through the origin with a slope 1/2A; this line is the locus of all points corresponding to 0:7. The periodic structure has an upper cutoff wave length (lower cutoff frequency) A when v,.Z =1r and a lower cutoff wave length A when 0:0. The curves thus cross line D at a point corresponding to A and pass through 0 at a point corresponding to A The portion of each curve on one side of line D represents amplifier operation while the portion on the other side of the line represents oscillator operation.

It can be shown that the attenuation u of the periodic structure, which is proportional to its characteristic impedance, is proportional to the reciprocal of the group velocity V The latter, in turn, is given by 0 rapi a V c V dh Tangents to points on these curves corresponding to some given value of A will intercept c/V aXis at a point whose coordinate is c/V for direct or forward waves and -c/V for backward waves.

It has been found that the upper cutoff wave length A in an interdigital delay structure increases as the distance 6 between the continuous electrically conductive surface of the backing member of the interdigital structure and the fingers of said structure increases.

In FIG. 11, curves A and B are shown for two values of 6. As 6 is increased, the upper cutoff wave length A increases from A to A The effect of variation of 5 on the lower cutoff wave length A has been observed to be comparatively slight so that X is substantially the same for both curves.

As 6 is increased, it is apparent from the curves that the slope of the curves decreases. For a given wave length a tangents drawn to curve A at points a and a intersect the ordinate c/V at points e and 2' corresponding to the ratios c/V and -c/V all respectively. The absolute values of c/V and c/ V are equal because of the symmetry of curve B about line D. Similarly, tangents drawn to curve B at points b and b intersect the ordinate c/V at points f and f corresponding to the ratios c/V and c/V all respectively. Again, the magnitude of c/V and c/V are equal. As the value of 5 increases, it is evident from FIG. 11 that the magnitude of the ratio c/V also increases. Since the attenuation a-is proportional to the reciprocal of V and since 0 is a constant, an increase in 6 is thus accompanied by a decrease in attenuation of the periodic structure.

Referring now to FIG. 1, a traveling wave oscillator 2i) is shown incorporating a linear periodic anode structure 21. The anode structure 21 includes a continuous electrically conductive backing member 22 forming one of the walls of an evacuated envelope, which further includes an oppositely disposed wall 24, end walls 25 and 26 and a pair of side walls, not shown. Backing member 22 includes a raised portion 28 of reduced width, integral therewith, whose surface 39- will be referred to as a back wall. A pair of electrically conductive inter-digital assemblies 31 and 32 are attached, as by screws 33, to backing member 22 on opposite sides of raised portion 2?, as shown in FIGS. 2 and 3. Extending transversely to the longitudinal axis of the interdigital assembly 31 are a plurality of fingers 35 which extend almost to the opposite assembly Likewise, a plurality of transverse fingers 36 extend from the assembly 32 toward the assembly 51. The inner conductor 39 of a coaxial output coupling device 49 extends through an aperture 41 in backing member 22 and is attached to the first or end finger of interdigital assembly 31.

Positioned adjacent the output end of the periodic anode structure 21 is a cathode structure 43 having an electron emissive surface 44-. Cathode 43 is supported by a hollow supporting cylinder 45 which extends through an aperture in wall 24 of the tube envelope. Cylinder 45 is rigidly supported With respect to wall 24 by virtue of its connection to a metallic member 46 which surrounds cylinder 45 and, in turn, is sealed to wall 24 adjacent the aperture through which supporting cylinder 45 passes. A central conductor 47 extends through supporting cylinder 45 and is connected to one end of a heater coil, not shown, positioned in thermal proximity to cathode emissive surface 44. The details of this cathode are more specifically set forth in an application for Letters Patent, Serial No. 255,499, of Dench, file-d November 8, 1951, now Patent No. 2,809,328.

An auxiliary electrode 48 is positioned substantially parallel to the anode structure and spaced therefrom, as shown in FIG. 1. Electrode 48, which is otherwise referred to as a sole, is a trough-shaped member whose bottom surface is positioned somewhat lower than the electron emissive surface 44 of cathode 43. Sole 48 is supported relative to the remainder of the tube structure by means of a pair of supporting rods 49 rigidly attached to the bottom of the sole. These rods are insulatedly supported with respect to Wall 24 by means of metallic members 50 in turn sealed to ceramic seals 51. The latter are connected to conductive cylinders 52 which surround rods 49 and are, in turn, attached to recesses in wall 24 surrounding the apertures through which rods 49 pass.

Positioned beyond the opposite end of sole 43 and in substantial alignment therewith, is a collector electrode 54 which is rigidly supported by means of a lead-in rod 55 extending through an aperture in wall 24 and spaced therefrom. Rod 56 is supported relative to wall 24 by means of a conductive cup as, a ceramic cylinder 57, a metallic cylinder 58 surrounding rod 56 and sealed together in similar manner to the sole-supporting devices previously described.

An electric field may be established between the anode and the sole by connecting a source of direct current voltage, not shown, therehetween. The cathode 43 is negative with respect to the anode 21 but may or may not be at the same potential as the sole.

A transverse magnetic field is produced in the space between the periodic anode structure and the sole in a direction perpendicular to the electric field or in a direction normal to the plane of the paper. By proper adjustment of the magnetic field, the electrons emitted from the cathode will be directed a path adjacent the fingers of the periodic structure. interaction of the elec tron beam with a backward wave traversing the periodic spaces"? anode structure will result in the generation of oscillation in the tube.

In traveling wave oscillators, as previously mentioned, it is desirable to introduce attenuation at the end of the tube remote from the output en In order for the oscillator to operate satisfactorily over a broad band of frequencies, it is desirable to progressively increase the attenuation introduced so that a point along the region of attenuation will always occur at which cutoff is approached, regardless of the frequency. When the cutoff is approached, the attenuation will approach infinity and substantially all the incident power will be absorbed in a relatively short distance.

In order to progressively increase attenuation as the end of the tube remote from the output end is approached, the spacing 5 of the interdigital fingers from the back wall, usually relatively large, is gradually reduced in accordance with the previous discussion of FIG. 11. The portion 30' of back wall 30 between points X and Y is tapered, as shown in FIG. 1 and in the enlarged view of FIG. 4. The tapered portion 30 may be linear or may vary progressively in a nonlinear manner, such as exponentially, depending upon the type of attenuation characteristic desired.

Since the amount of attenuation produced is substantial only when the distance 6 is relatively small, it is possible to construct the back wall with a multiple taper. In FIG. 5, a backing member having a doubly tapered back wall is shown. The first portion 30" between points W and X serves to rapidly decrease 5 so that over the portion 39, corresponding to portion 30' of FIGS. 1 and 4, the attenuation per unit length is greater than that produced by the single taper of FIG. 4.

In FIG. 6, a tapered periodic anode delay structure for use in a traveling wave amplifier is shown. The elements of the tube of FIG. 6 corresponding to the elements of the tube of FIG. 1, are indicated by like reference numerals. Since the tube of FIG. 6 is an amplifier, two energy coupling means 40 and 49 must be provided in the device of FIG. 6. A radio frequency signal is ap plied by way of connector 4% and output energy is derived by means of output connector 40. If the output of the amplifier is improperly matched to the load, a refiected wave from the output end will travel back to the input end. Reflections further occur from the input end and may cause positive feedback and undesirable 0scillations. The tendency to oscillate is reduced by introducing attenuation over a region intermediate the input and output ends, thereby minimizing the amplitude of the wave reflected from the input end.

If the attenuation is introduced too near the input end where the signal is small in amplitude, the level of the signal beyond the attenuating region will be unduly small, if not negligible, and the amplification factor of the tube will be severely reduced. If, on the other hand, the attenuation is introduced too far from the input end, the efficiency of the tube is considerably reduced because of the relatively large amount of power dissipated in the attenuating region. In practice, it has been found desirable to insert the attenuation approximately one-third of the distance from the input end of the anode structure. In. this way, the transmission loss in the wave transm' sion path is not sufficient to seriously affect the amplifi cation of the tube but is adequate for suppressing the reflected wave energy traveling in the reverse direction. The desired attenuation is introduced by means of a tapered portion 39 between points P and Q of back wall 3%; this taper will obviously be in the opposite direction from that of FIG. 1.

It is sometimes desirable to taper the back wall 3t in both directions, as shown in FIG. 7, so that energy refiected from the input end also will be reduced. In this case, portion 30a is tapered in the manner shown in FIG. 7. The taper of portion 3011 need not be as pronounced as that of portion 30' since the energy level at the input end of the tube is much lower than that of energy refiected from the output end after amplification. T :e portion 34) from points P to Q in FIG. 7 corresponds to the portion St? between points X and Y of FIG. 1 and, like the latter, is not restricted to a linear taper.

In FIGS. 14 and 15, the traveling wave oscillator is shown using an interdigital delay line of circular configuration. This basic tube is the subject of an application for Letters Patent, Serial No. 357,824, of "Dench, filed May 27, 1953, now Patent No. 3,027,483. Briefly this tube comprises an arcuate interdigital structure 60 having a backing member 61 whose inner surface 7t) will be referred to as a back wall. The backing member, as shown here, forms part of the tube envelope, although it may be attached to the envelope by appropriate fastening devices. The interleaved electrically conductive fingers 62 and 63 are spaced from said back wall by a distance 6. Fingers 62 and 63 are also separated from the outer wall of a cathode-sole assembly 65, shown here for simplicity only as a circular member, to form the necessary interaction space 66 which is traversed by an electron beam produced by the cathode. The cathode electrical connections may be brought out through the center of an insulating supporting member 75.

The tube is closed by means of cover plates 67 and 68, a portion of which is shown in FIG. 15. The cover plates may be attached to the backing member 61 by screws 69. A magnetic field transverse to the direction of flow of the electrons is produced by means of a magnet, one pole 71 of which is shown in FIG. 14. An electron intercepting electrode or connector 76 is provided at one end of the anode structure for preventing electrons in the beam from entering the area adjacent said other end of the anode structure. An output coupling means 72 is provided at one end of the tube and includes an outer conductor 73 and an inner conductor 74, the latter of which is inserted through an aperture in backing member 61 and is connected to one of the end fingers of the interdigital structure.

Attenuation is introduced into the other end of the interdigital anode structure. This attenuation includes a lossy material, not shown, applied to the fingers of the anode structure at said other end. As in the case of the linear traveling wave tube previously described, the distance 6 between th back wall 7t) of the anode structure and the fingers 62 and 63 is progressively decreased as the end of the anode structure remote from the output coupling means is approached. The portion 7th., between points X and Y is gradually tapered to provide the desired attenuation, in like manner to the device of FIG. 1.

In FIG. 13 a portion of a traveling wave tube of the nonmagnetic type, that is, a tube having no transverse magnetic field associated therewith, is shown using an interdigital structure comprising a first series of fingers 50 connected to one wall 81 of the tube envelope and a second series of fingers 82 connected to the opposite wall 83. An electron beam, which is generated and focused by means of a conventional electron gun, not shown, is directed in the direction of the arrow in FIG. 13. The walls 81 and 83 in which the interdigital fingers are attached form a portion of the tube envelope, or alternately may be enclosed within an evacuated structure. The fingers each contain an aperture 84 through which the electron beam is passed. In this tube, both walls 36 and 87 have a portion 86' and 87', respectively, tapered so that the distance 5 between the tapered wall and the adjacent edge of the fingers is progressively decreased. In this way, the desired gradually increasing attenuation is introduced over said portion. The direction of the taper will, of course, depend upon whether or not the tube is being used as an oscillator or an amplifier.

In FIG. 8, a traveling wave tube is shown utilizing a loaded wave guide and a periodic anode structure instead of the interdigital delay structure previously described.

This loaded wave guide structure, which is the subject spa es? J of US. Letters Patent No. 2,888,609 issued to lohn E. Smith, on May 26, 1959, is shown in FIGS. 9 and 10 and consists of an open wave guide 90 loaded at the open face by a plurality of half wave length shorted loops or straps 91 uniformly spaced along the length of the structure. The wave guid includes a back wall 98, and a pair of side walls 99 and 100. The open wave guide is shown as integral with an electrically conductive face 93 which forms one wall of an evacuated envelope further including walls 94, 95 and 96 and a pair of end walls, not shown. Alternately, the Wave guide could be attached to the wall or face 93 of the tube by appropriate fastening means.

The loading straps are substantially U-shaped loops whose ends are pressed into slots 97 in base 93 and fastened by brazing. Alternate straps are brazed or otherwise secured to opposite side walls 99 and 1%, respectively, of the wave guide.

The tube of FIG. 8 utilizing the periodic structure shown in FIG. 10 is an oscillator and includes a sole L92 supported from one of the walls 96 of the tube by an insulatedly supported member 103. An electron beam 105 travels the interaction space between anode and sole.

A transverse magnetic field is produced by means of a magnet whose pole pieces 96 and 97 are shown in FIG. 8.

It has been found that the lower cutoff wave length of the loaded Wave guide periodic structure is a function of the width H of the wave guide. As the width of the guide is increased, the lower cutoff Wave length AL is increased from A to )t as shown in FIG. 12. The upper cutofi. wave length is not affected appreciably by changes in value of H, however, since the low frequency cutoif is determined by the electrical length of the loops. The effect on the ratio c/V of an increase in H is portrayed graphically in FIG. 12 in which curves K and M correspond, respectively, to lower and higher values of H. A tangent to curve K at point k and k at some wave length i intersects the ordinate of the curve at a point r and r corresponding to a ratio c/V The tangent to curve M at points m and m corresponding to A indicates a ratio c/V In other words, as the width H is increased, the reciprocal of V increases. The attenuation a which from equation (2) is directly proportional to 1/ V increases for increasing values of H. in FIG. 8, the width H of the wave guid '90 is gradually increased over the portion 98 beyond point X. This tapered width is positioned at the end of the tube in the example shown in FIG. 8. If the loaded wave guide anode structure is to be used in a traveling wave amplifier, tapered portion 98 would be positioned intermediate the input and output ends of the tube in accordance with the previous discussion of amplifiers.

Although the loaded wave guide anode is shown in connection with a traveling wave tube of the magnetic type, it may be used equally as well in a traveling Wave tube having no transverse magnetic field.

It has been found that changes in the dimension 5, that is, the distance between the back Wall 108 of the wave guide and the loops 91 of the wave guide structure, also has an effect upon the upper cutoff wave length A similar to that of the interdigital line. The curves of F113. 11 are thus also applicable to the loaded wave guide periodic structure. The effect of 5 on M, however, is considerably less in the case of the loaded wave guide. The desired attenuation obviously may be introduced by increasing the Width H of the wave guide of FIG. 8 simultaneously with an increase in the spacing 8.

It should be understood that the interdigital and loaded wave guide anode structure are interchangeable and may be used in any of the tubes shown herein, whether linear or circular, magnetic or nonmagnetic, or amplifiers or osciliators. The region of the eriodic anode structure over which attenuation is effected in accordance with this invention may also contain a coating of lossy material or 8 may be constructed of lossy material to further enhance the attenuation obtained. For example, the fingers of the interdigital line may be coated with graphite, plated with iron or actually constructed of iron.

This invention is not limited to the particular details of construction, material and processes described, as many equivalents will suggest themselves to those skilled in the art. It is, accordingly, desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claim d is:

1. A travelling wave oscillator comprising an evacuated envelope, a periodic structure mounted within said envelope for propagating electromagnetic wave energy, said periodic structure including an interdigital delay line having a continuous electrical conductive member loaded by a plurality of interleaved electrically conductive fingers uniformly disposed along a portion of said member and spaced therefrom, means connected to one end of said structure for deriving an output, a source of electrons,

and means for directing said electrons along a path adjacent said periodic structure and in energy interacting relationship with said wave energy, said portion of said member at one end of said structure having at least one dimension altered to progressively decrease the distance between said member and said fingers as said one end is approached for introducing a gradually increasing attenuation in said portion.

2. A travelling wave oscillator comprising an evacuated envelope, a periodic structure having an output end and mounted longitudinally within said envelope for propagat-- ing radio frequency wave energy with a predetermined group velocity toward said output end, said periodic structure including a continuous electrically conductive memher and a plurality of lumped circuit elements cooperating with and uniformly disposed along a portion of said member, said periodic struction further having an electromagnetic field associated therewith comprising a plurality of traveling space harmonic waves, a first set of which travel in a first direction corresponding to the direction of flow of said wave energy and a second set of which travel in a second direction opposite said first direction, a source of electrons, and means for directing said electrons in said second direction along a path adjacent said periodic structure substantially in synchronism with and in energy interacting relationship with one of said second set of space harmonic waves, the spacing between said lumped circuit elements and electrically conductive member toward the end thereof electrically remote from said output end being progressively decreased over said portion for gradually increasing the attenuation in said portion.

3. A travelling wave oscillator comprising an evacuated envelope, a periodic structure having an input and an output end and mounted longitudinally within said envelope for propagating ratio frequency wave energy with a predetermined group velocity toward said output end, said periodic structure including a continuous electrically conductive member and a plurality of lumped circuit elements cooperating with and uniformly disposed along a portion of said member, said periodic structure further having an electromagnetic field associated therewith comprising a plurality of travelling space harmonic waves, a first set of which travel in a first direction corresponding to the direction of fiow of said wave energy and a second set of which travel in a second direction opposite said first direction, means for producing a magnetic field normal to the direction of flow of said Wave energy, a source of electrons, and means for directing said electrons in said second direction along a path adjacent said periodic structure substantially in synchronism with and in energy interacting reiationship with one of said second set of said space harmonic waves, said portion of said periodic structure member adjacent the end thereof electrically remote from said output end being spaced progressively closer to said lumped circuit elements to thereby progressively increase attenuation toward said electrically remote end of said periodic structure so as to substantially eliminate wave reflections from said remote end.

4. A crossed-field traveling wave tube including a delay line conductively attached to an elongated anode structure, an electrode coextensive with said delay line defining a beam interaction space therebetween, means injecting electrons into one end of said interaction space, and means collecting electrons at the other end of said interaction space, the spacing between said elongated anode structure and one end of said delay line being gradually varied to thereby vary the impedance to radio frequency waves.

References Cited in the file of this patent UNITED STATES PATENTS Warnecke et al Aug. 31, 1954 Ginzton Dec. 28, 1954 Pierce May 10, 1955 Epsztein Mar. 31, 1959 Kompfner July 14, 1959 Karp July 19, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORECTION Patent No, 3 O69 587 December 18 1962 Edward C Dench It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8 line 6 for "material" read materials line 48, after end insert said -g line 56, for "ratio" read radio e Signed and sealed this 28th day of May 1963,

(SEAL) Attest:

ERNEST w. SWIDEE. DAVID LADD fit-testing @fficer Commissioner of Patents 

